pantsbuild / pants / 35257

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Filter log levels by target (#10729)

### Problem

Pants's logging, particularly on the DEBUG and TRACE levels, is too verbose to make effective use of. We would like to be able to selectively enable more verbose logging for specific areas of the pants codebase where we think a problem we're trying to debug might lie, while leaving others at the default logging level.

### Solution

The Rust and Python logging frameworks that Pants uses already have the concept of a *target*, that is, a name associated with a log object that comes from the area of the codebase where that log is emitted. For Python logs, that target name is a fully-qualified Python module (e.g. `pants.pantsd.service.pants_service`) and for Rust logs it is the name of the Rust module (e.g. `workunit_store`). 

This commit adds a new global option `--log-levels-by-target` that expects a dictionary mapping strings representing these target names to the string representation of a Pants log level. By this mechanism, a user can specify that logs originating in a given part of the pants codebase will be subject to less filtering than other logs. Log targets not explicitly specified in this option are filtered according to the global log level specified with `--level`. 

This commit also adds another new global option `--show-log-target` that simply adds the target name to log messages without doing any filtering, to make it easier for users casually inspecting logs to see what log targets might be relevant for further debugging.

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